The properties and characteristics of lasers are now known and techniques for stabilizing laser systems have likewise been heretofore suggested and/or developed. One such stabilizing technique is described, for example, in "Laser Frequency Stabilization by Means of Saturation Dispersion", by G. Kramer, C. O. Weiss and J. Helmcke, Z. Naturforsch. Teil A:30,1128 (1975) wherein a 3.39 .mu.m He-Ne laser was frequency stabilized utilizing a dispersion frequency stabilization scheme which included an external stabilized reference laser (as a frequency reference for heterodyne detection of frequency pulling effects) and a reversible counter in the control system. Performance, however, was degraded due to frequency noise intrinsic to the use of a separate laser for the heterodyne reference function.
The properties and characteristics of two-mode lasers, such as Zeeman lasers wherein the two modes are symmetrically disposed around the atomic line center, are also now known, and frequency stabilization techniques have likewise been heretofore suggested for such lasers.
Frequency stabilization of the Zeeman laser is shown, for example, in U.S. Pat. No. 3,534,292. In this system, the cavity of a two frequency Zeeman-split laser is modulated in length by a piezoelectric element to produce a modulated difference frequency that is discriminated to provide a signal having AC and DC components with the AC component being detected to provide an error-correction signal that is coupled to the piezoelectric element, and with the DC component being fed back to the laser to control the magnetic field applied to the laser.
It is has also been heretofore suggested that a heater wrapped around the structural elements of the resonator of a laser could utilize the control voltage from a cavity length servo to cause the heater to be switched off and on for thermally controlling cavity length so that the main piezoelectric servo continually operates about the center of the dynamic range ["Laser Frequency Stabilization: Combined Integrating Thermal-Proportional Servos", Clark, Applied Optics, Volume 15, No. 6, page 1375 (1976)].